Serotonergic projections from the nucleus raphe dorsalis to the amygdala in the rat.

Previous studies have shown a possible connection between the nucleus raphe dorsalis (NRD) and the amygdala in mediating opioid analgesia. In the present study, horseradish peroxidase (HRP) retrograde tracing was used in combination with serotonin (5-HT) immunocytochemical staining in an attempt to search for serotonergic projections from the NRD to the amygdala. In rats which received an injection of HRP into the amygdala, HRP retrogradely labelled 5-HT-immunoreactive cells were observed in the NRD. About 10% of the 5-HT-immunoreactive neurons in the NRD give rise to axons to the amygdala. These cells are predominantly situated in the ipsilateral wing and ventromedial part of the NRD. These data indicate the existence of serotonergic projections from the NRD to the amygdala, providing a morphological substrate for the putative antinociceptive pathway from the NRD to the amygdala.

Alterations of immunoreactive substance P and enkephalins in rat spinal cord after electroacupuncture.

A form of electrically-induced analgesia known as electroacupuncture was administered to rats bilaterally at the point "Huan-tiao." Compared with untreated rats, treated rats showed altered pain thresholds characterized as low, intermediate, and high. From immunocytochemical studies, the spinal cords taken from the treated rats exhibited differences in immunoreactivity for substance P (SP), methionine- and leucine-enkephalins (ME and LE respectively). By densitometry, the altered levels of immunoreactive (IR) peptides correlated with the pain thresholds in specific ways. That is, high pain threshold correlated with the visualization of increased IR-SP adn IR-LE within neuronal processes throughout the dorsal horn substantia gelatinosa. In the same specimens, decreased IR-ME could be seen. In contrast, low pain threshold correlated with decreased IR-SP and IR-ME. IR-LE showed a concomitant decrease in the medial substantia gelatinosa region, and slight, insignificant changes laterally. The data suggest that different degrees of analgesia induced by electroacupuncture result from the variable release of SP, ME, and LE in spinal regions associated with nociception. In terms of current models of pain processing, the data do not entirely support an axo-axonic interaction between enkephalin interneurons and SP terminals. Some modifications and an alternative model are considered.